Optical elements often contain subsurface damage such as cracks, voids, or contaminant particles. This subsurface damage may be inherent to the type or quality of the material used in the optical element or may be produced during fabrication processes such as sawing, grinding, and polishing. Regardless of the source, the amount of subsurface damage in precision optical elements often needs to be evaluated because the subsurface damage can affect the performance of the optical element. In particular, subsurface damage in an optical path of an optical element can cause light absorption and scattering that may lead to heat generation or a poor quality optical signal.
Non-destructive techniques that measure the surface profile of optical elements are known but generally do not provide sufficient information about the subsurface damage.
One technique for evaluating the subsurface damage in an optical element is to mathematically model the optical element based on knowledge of the materials and fabrication processes employed. With an accurate model, the expected damage in a particular design can be determined. However, such models will be inaccurate if the model is based on inaccurate characterizations of the material or the fabrication process or if effects not anticipated in the model cause damage. Further, even when a model is accurate, the level of subsurface damage in individual optical elements is generally subject to variations, and a specific optical element may have more or less subsurface damage than expected.
A number of destructive evaluation techniques are available for measurement of subsurface damage. A taper grinding process, for example, can cut into samples of an optical element design to expose the subsurface damage for evaluation. Alternatively, damage at an optical surface can be enlarged or exposed using an acid etch. The acid etch generally has greater effects on cracks, thereby permitting surface examination with a high power microscope to detect the enlarged or exposed cracks or defects. These destructive techniques avoid the need to accurately model materials or the fabrication processes used in an optical design. However, each optical element evaluated in this manner is generally destroyed or rendered unusable, which is very undesirable for expensive systems or elements. Also, destructive evaluations only measure typical subsurface damage for a design. The problem of variations in optical elements that are not directly evaluated leaves uncertainty in the amount of subsurface damage in the useable optical elements. Further, the results of a destructive evaluation can typically take several days to obtain.
Acoustic measuring techniques, for example, combining Hertzian Acoustic Emission Indentation (HAEI) and a Line Focus Acoustic Microscope (LFAM), can examine the surface condition and fracture toughness of an optical element. However, such techniques have difficulties in detecting defects larger than about 10 μm and also require expensive equipment that requires a significant amount of skill, training, and support to operate.
Efficient techniques for non-destructive evaluation of subsurface damage in optical elements are accordingly sought.